The invention concerns a process and an apparatus for testing resilient elements.
The expression "resilient elements" refers to elements having a resiliency over a certain spring range. These include elements that serve for spring loading such as coil springs, rubber springs, leaf springs or the like, and at least partly, resilient elements serving an additional or different purpose such as shock-absorber legs for motor vehicles or skis for alpine, jumping or cross-country skiing.
For developing such elements for final control in the production or for vendor inspection of external parts, the elements must be dynamically tested under load conditions close to those existing in use. This means that the elements are exposed, for example, to an endurance test of usually up to about 10.sup.6 load cycles.
In one kind of formerly used test bench (East German Patent No. 656), there are provided eccentric drives to introduce test deformations in the clamped resilient elements. This kind of drive has, on one hand, the disadvantage that the testing frequency must be relatively low since, otherwise, the forces due to inertial masses that cannot be balanced lead to bearing loads, etc. that can no longer be neglected. The low frequency of the load reversal, in turn, leads to an extremely long testing time on the individual part which is a hindrance for the development of such elements and considerably delays sample testing of production parts. Another problem in testing benches with eccentric drives is that only oscillation tests with constant amplitudes can be performed since the adjustment of the height of the eccentric drives is extremely difficult, and can only be carried out while the apparatus is idle.
To obtain stress profiles close to in use condition in which a plurality of amplitudes are used, servo-hydraulic test equipment is known. Servo-hydraulic test equipment is not only extremely expensive to produce, but also has a high wear and power requirement, so that operation is very costly. Especially in the testing of leaf springs (such as for trucks), just the cost of operation of the equipment in a servo-hydraulic system is extremely high. As one example makes clear, for testing a 10 ton leaf spring with 300 mm stroke and 10.sup.6 stress cycles which are passed at a frequency of 2 Hz, the cost of operation falls in the range of about 20,000 German Marks, since the hydraulic energy applied in spring loading is converted to heat during spring unloading.
German Patent Application No. 25 22 890 has further disclosed servo-hydraulic test equipment where an auxiliary weight is attached to a coil spring to be tested so that a servo-hydraulically excited oscillation system results. Although this arrangement has a clearly lower consumption of energy, oscillations are excited on the show place, which is not only disturbing and requires an expensive bench, but also represents a loss of energy.
The principle of material testing which makes use of the test sample in an oscillation system is also known from German Patent Application No. 25 33 373 or U.S. Pat. No. 4,539,845. However, with both known systems, only oscillations with very low amplitudes can be excited, thus requiring the use of springs having a large displacement.
East German Patent No. 216,317 discloses a process wherein a mass element is introduced between two coil springs placed in a row and fastened by the ends, so that a series oscillatory circuit results. The mass element is reciprocally moved directly over a linear drive at the resonant frequency of the system. In this arrangement, the whole system is dislocated in oscillations and thus, the above described disadvantages result.
Thus, the problem, as a whole, is that all known processes or systems entail high costs of operation and are limited in their working speed (resonant frequency) as a result of the losses caused by the system.
The periodical "Soviet Inventions Illustrated", Week B40, Nov. 14, 1979, No. J5481 B/40, London, G.B., discloses a process and an apparatus for testing resilient spring elements which are supported between two lever beams and are in opposed phase so as to be offset in their oscillations. This phase opposition of the levers is obtained by gear connections between the levers. Due to this connection between the levers, the arrangement is, on one hand, inflexible regarding the adjustment for receiving different test samples and, on the other hand, with the gear connection, oscillations are produced that adversely affect the results of the test. Finally, a very high torque has to be applied when starting up the installation.
FR-A 2,292,966 discloses a system wherein two springs are clamped between both ends of a lever and a bench. The arrangement is to be operated by resonant frequency. As described above, such an arrangement is objectionable insofar as forces of inertial masses considerably limit the testing frequency and also the testing amplitudes.
Based on the above prior art, the problem solved by this invention is to develop a process and an apparatus of the above mentioned kind in the sense that resilient elements can be tested quicker than formerly accomplished and in a simple, inexpensive way.